1. Field of the Invention
The present invention generally relates to a circuit electrode and a method of forming a circuit electrode. More particularly, the present invention relates to a circuit electrode formed on a package of an integrated circuit or on a printed circuit board of the same, as well as to a method of forming such a circuit electrode.
2. Description of the Background Art
A ball grid array (BGA) package and a chip scale package (CSP) have already been known as packages suitable for meeting a demand for an integrated circuit having an increased number of pins. A circuit electrode coated with a metal pattern, such as a Cu pattern, by means of electroless plating is used for such a package.
FIG. 6 is a cross-sectional view showing a conventional integrated circuit package which is equipped with a circuit electrode having the above-described structure. The conventional integrated circuit package has an organic substrate 10. A pattern 12 is formed on the organic substrate 10 from metal, such as Cu. The metal pattern 12 is coated with an electroless Ni--P plating film 14 (hereinafter referred to as a "Ni--P film 14"), and the Ni--P film 14 is further coated with an immersion gold plating film 16 (hereinafter referred to as an "Au film 16"). On the surface of the organic substrate 10 is formed a passivation film 18 in order to prevent oxidation of the metal pattern 12 or flow of solder from the circuit electrode.
The Ni--P film 14 is an electroless plating film containing Ni as a major constituent and P in an amount of about 6 to 8 wt. %. This Ni--P film 14 is provided primarily for the purpose of preventing diffusion of Cu, which constitutes the pattern 12. The Au film 16 is an electroless plating film formed on the Ni--P film 14 by means of the substitution plating technique and is provided primarily for the purpose of preventing oxidation of the Ni--P film 14. In the above-described configuration, a circuit electrode is formed from the pattern 12, the Ni--P film 14, and the Au film 16. The circuit electrode is connected to an electrode of a printed circuit board by way of a solder ball or bump to be formed on the Au film 16 or by a bonding wire to be bonded on the same.
An electroless plating film of Au can be formed by the substitution plating technique or the reduction plating technique. However, if the reduction plating technique is employed for forming the Au film on a Ni--P film 14, existence of Ni ions hinders growth of the Au film. For this reason, the Au film 16 must also be grown by means of the substitution plating technique, before employing reduction gold plating, as mentioned previously.
The substitution plating technique is a method of growing an Au film by substituting Au for Ni contained in the Ni--P film 14. Since P is not substituted by Au at this time, Au is first deposited on the P-free areas of the surface of the Ni--P film 14 during the course of plating of the Au film 16. Accordingly, during the course of plating of the Au film 16, the lower the P concentration in the Ni--P film 14, the faster the rate of substituting reaction between Ni and Au.
In other words, if the P concentration in the Ni--P film 14 is too high, substitution of Au for Ni is not sufficiently carried out, and pinholes are likely to arise in the Au film 16. In the event that pinholes arise in the Au film 16, oxidation of the Ni--P film 14 cannot be prevented, thus wire bonding or soldering without use of flux becomes difficult. For this reason, the P concentration in the Ni--P film 14 must not be set excessively high.
In the conventional circuit electrode, the P concentration in the Ni--P film 14 assumes a value of 6 to 8 wt. %. In other words, P ions occupy about 6 to 8% portion of the surface area of the Ni--P film 14. During the course of plating of the Au film 16, erosion of Ni by Au proceeds in the vertical direction while Ni is substituted by Au in the areas where P ions do not exist. If the rate of reaction between Au and Ni is high, erosion of Ni by Au becomes noticeable, resulting in formation of crack-shaped eroded portions in the Ni--P film 14.
The lower the P concentration of the Ni--P film 14, the faster Au ions react with Ni ions. For example, if the P concentration of the Ni--P film 14 assumes a value of 3.5 wt. %, Au is deposited to a thickness of 0.5 .mu.m when the Au film 16 is plated for 10 minutes. In a case where the P concentration assumes a value of 7.5 wt. %, Au is deposited to a thickness of 0.3 .mu.m when the Au film 16 is plated for 10 minutes. Further, in a case where the P concentration assumes a value of 9.5 wt. %, Au is deposited to a thickness of 0.05 .mu.m when the Au film 16 is plated for 10 minutes. Thus, the lower the P concentration of the Ni--P film 16, the greater the amount of Ni ions that are substituted by Au ions during the plating process. As a result, crack-shaped eroded portions are likely to arise in the Ni--P film 14.
The areas of the Ni--P film 14 where Ni ions are substituted by Au ions are higher in P content than are other areas. At this time, the greater the amount of Ni ions substituted by Au ions, the higher the P concentration. Accordingly, the lower the original P content in the Ni--P film 14, the higher the differential in P content in the boundary region between the Au film 16 and the Ni--P film 14 after the Au film 16 has fully grown.
FIG. 7A shows a solder ball 20 bonded to the circuit electrode shown in FIG. 6. FIG. 7B shows removal of the solder ball 20 from the circuit electrode as a result of a shear test. In FIGS. 7A and 7B, like reference numerals are assigned to elements which are identical with or correspond to those shown in FIG. 6, and repetition of their explanations is omitted.
The solder ball 20 is formed from eutectic solder containing lead (Pb) and tin (Sn). During the course of the solder ball 20 being soldered to the circuit electrode, the Au film 16 covering the Ni--P film 14 becomes fused in the solder ball 20. As a result, the solder ball 20 comes into direct contact with the Ni--P film 14. After the solder ball 20 has come into contact with the Ni--P film 14, Sn contained in the solder ball 20 react with Ni contained in the Ni--P film 14 to form an Ni/Sn compound 24 (Ni.sub.3 Sn.sub.4).
During the course of generation of the Ni/Sn compound 24, the diffusion rate of Ni is reduced by the presence of P ions in the P-containing areas on the surface of the Ni--P film 14. If any areas of the surface of the Ni--P film 14 have a high P content, therefore, the Ni/Sn compound 24 is likely to assume a columnar profile as shown in FIGS. 7A and 7B. Further, since P ions are left in the Ni--P film 14 during the course of generation of the Ni/Sn compound 24, a P-rich layer 22 whose P content is higher than that of the other areas is generated in the vicinity of the boundary region between the solder ball 20 and the Ni--P film 14.
As a result of the Ni/Sn compound 24 acting as an adhesive, the solder ball 20 is soldered to the Ni--P film 14. The P-rich layer 22 generated between the Ni/Sn compound 24 and the Ni--P film 14 acts as a contaminated layer and weakens the adhesive action of the Ni/Sn compound 24. Formation of the P-rich layer 22 results in a decrease in bonding strength between the solder ball 22 and the Ni--P film 14, which in turn renders the P-rich layer 22 susceptible to removal, as shown in FIG. 7B.
As mentioned above, as erosion of the Ni--P film 14 associated with growth of the Au film 16 becomes more noticeable, the P concentration on the surface of the Ni--P film 14 increases. In other words, as the erosion of the Ni--P film 14 associated with growth of the Au film 16 becomes more noticeable, the P ions existing in the Ni--P film 14 tend to become more concentrated. As the P ions existing on the surface of the Ni--P film 14 become more concentrated at the time of bonding of the solder ball 20, the P-rich layer 22 becomes more likely to be formed. Consequently, as the Ni--P film 14 becomes more noticeably eroded in association with growth of the Au film 16, the bonding strength between the solder ball 20 and the Ni--P film 14 becomes more likely to drop.
The portion of the Ni--P film 14 that is considerably eroded by Au ions has a greatly reduced Ni content. For this reason, when the solder ball 20 is bonded to the circuit electrode, the Ni/Sn compound 24 is less likely to be formed in the eroded portion. Even in this respect, if the Ni--P 14 film is eroded noticeably by Au ions, the bonding strength between the solder ball 20 and the Ni--P film 14 is likely to decrease. Accordingly, in order to ensure the bonding strength exerted on the solder ball 20, erosion of the Ni--P film 14 caused by Au ions must be prevented.
As mentioned above, in order to prevent formation of pinholes in the Au film 16, the P concentration in the Ni--P film 14 cannot be set excessively high. In contrast, in order to prevent erosion of the Ni--P film 14 caused by Au, the P concentration cannot be set excessively low. In consideration of these two conflicting requirements, the Ni--P film 14 has conventionally had a P content of 6 to 8 wt. %.
However, the conventional Ni--P film 14 can sufficiently prevent neither erosion of the Ni--P film 14 caused by Au, nor generation of the P-rich layer 22 caused by soldering. For example, in a case where the Au film 16 is provided to a thickness of about 0.2 .mu.m on the surface of the Ni--P film 14 containing P at a concentration of 7.5 wt. %, the P concentration assumes a value of 10.2 wt. % within a depth of about 1 .mu.m from the surface of the Ni--P film 14. Further, if the solder ball 20 is bonded to the Au film 16, the P concentration in the areas immediately below the Ni/Sn compound 24 assumes a value of 17.5 wt. %.
Thus, in the conventional circuit electrode using the Ni--P film 14 having a P content of 6 to 8 wt. %, the area between the solder ball 20 and the circuit electrode is still susceptible to solder failure. For example, in a case where a circuit electrode having a diameter of 0.6 mm, and comprising the Ni--P film 14 containing P at 6 to 8 wt. % and the Au film 16 of 0.12 .mu.m thickness is formed on Cu pattern 12, the result of shear test were sometimes that the defective ratio which fractured between the solder balls 20 and the Ni--P film 14 is more than 40%.
Reducing the thickness of the Au film 16 is a conceivable measure for preventing erosion of the Ni--P film 14 caused by Au, i.e., for preventing generation of the P-rich layer 22. However, the thinner the Au film 16, the more susceptible the Au film is to formation of pinholes. For this reason, if the thickness of the Au film 16 is reduced, the organic substrate 10 must be etched immediately before being subjected to wire bonding or fluxless soldering, so as to eliminate the oxide film from the surface of the circuit electrode. Accordingly, reducing the thickness of the Au film 16 to prevent generation of the P-rich layer 22 results in disadvantages such as deterioration of reliability of the circuit and an increase of manufacturing cost of the same.